France did not "concentrate" them in specific areas (that old cliché). It's just for a variety of reasons (that were never part of a specific plan or policy) they ended up concentrated in specific areas.
These reasons include:
- too many fragmented municipalities (because France refuses municipality mergers unlike the rest of Europe, so where London has only 33 boroughs, Paris over the same urban territory has no less than... 244 independent municipalities!
), with many of these small municipalities being taken over by hard-left parties (whereas in larger London boroughs, which mix up more diverse populations over larger territories, it's hard for non-mainstream parties to win power), and these hard-left mayors (members of the French Communist Party in general) have deliberate policies of building more than 50% of social housing (i.e. more than 50% of dwellings in the entire municipality are social housing) to attract impoverished and working-class populations who will vote for them and keep them in power. More recently those hard-left mayors have pandered to the Islamists and what Québec would call the "ethnic vote".
- a glut of badly built social housing built cheaply in the 1950s, 1960s, and 1970s on the outskirts of almost all French cities when the French population was booming and lots of new housing was needed. Native French people have quickly left these social housing areas (called "cités" in France, in the US they would be called "projects") because the buildings have decayed, and the native White population has been replaced by mostly African populations (Blacks and Arabs). It's a natural process, not something that the authorities have ever devised or implemented.
- the fact that France's immigration is predominantly coming from Africa since the 1970s, whereas immigration in Germany or the UK was until recently coming predominantly from Europe, and people with a very different culture are harder to integrate than Europeans.
All of these combined lead to the disastrous situation of many French suburbs. It could only be improved if a- we massively merged our urban municipalities to root out extremist parties and have our large cities ruled by mainstream, centrist parties, and b- if we stopped African immigration (which the left and even the center in France consider "fascist" or "far-right"). The new French government to be announced before Monday should have a right-wing conservative as the new minister of the Interior, a guy who would be considered just an average Conservative in Canada or the UK, or an average Christian-Democrat in Germany, but here the left-wing media and parties, and even some centrists, are already saying that we allow a "hard-right" "arch-conservative" guy "whose values we don't share" to enter the government, as if he was some sort of Steve Bannon extremist.
This is France. Far too lax, far too left-wing, as usual. Here QS would be considered just mainstream left, the PQ already rightwing, and Legault probably super right-wing and "ultralibéral" (which is a great insult in French politics, think Thatcher as the quintessential "ultralibéral").